The present invention relates to methods of treating food products, more particularly, methods of heat-treating food products.
Contamination of shell eggs and egg products with Salmonella is a major public health concern. It is believed that Salmonella infection of shell eggs occurs either by transovarian transmission to the developing follicles or by trans-shell contamination from environmental sources (R. G. Board et al., (1996) J. Applied Bacteriology 29:319). Until recently, however, no effective and commercially-feasible methods for reducing or eliminating Salmonella in shell eggs have been available.
The greatest food safety challenge facing the shell egg industry is the now clearly established linkage between the consumption of undercooked shell eggs and outbreaks of salmonellosis caused by Salmonella enterica serovar Enteritidis (SE). This serovar first emerged as a public health challenge in Europe in the 1920s, where SE foodborne illnesses were frequently traced to the consumption of duck eggs (Tauxe, (1999) Salmonella Enteritidis: the continuing global public health challenge. In Saeed, A. M. (ed.) Salmonella enterica serovar Enteritidis in Humans and Animals: Epidemiology, Pathogenesis, and Control. Iowa State Univ. Press (Ames, IA)). In the United States, highly invasive strains of SE began to establish themselves within flocks of egg-laying chickens in the Northeast in the late 1970s. SE may contaminate the internal contents of the egg via a transovarian transmission route or, under certain conditions, by passage through the shell exterior. The first large, United States multi-state SE outbreak linked to shell eggs occurred in 1986. In 1988, St. Louis et al. published a breakthrough epidemiological review paper that documented that SE was much more likely to be associated with egg-containing foods than were other serovars, that the eggs involved were nearly always U.S. grade-A commercial shell eggs, that the eggs were sourced from many different flocks, and that they were typically not fully cooked (St. Louis et al., (1988) J. Amer. Med. Assoc. 259:2103). In the ensuing years, this problem has grown to the extent that outbreaks of egg-associated SE illness have been documented throughout the United States and in many other regions of the world, causing what the Centers for Disease Control and Prevention has characterized as a xe2x80x9cgrowing worldwide pandemicxe2x80x9d (Tauxe; supra). SE is also now the serovar most frequently isolated from patients with salmonellosis in the U.S. and in many nations around the world.
Since 1990, the U.S. Food and Drug Administration has designated grade A shell eggs as a xe2x80x9cpotentially hazardous food,xe2x80x9d due to their proven linkage to outbreaks of SE foodborne illness (FDA, (1990) Potentially hazardous Foodxe2x80x94shell eggs. Retail Food Protection Program Information Manual. FDA, Center for food Safety and Applied Nutrition. Washington, DC: U.S. Government Printing Office). SE has emerged as a significant public health concern for several reasons. First has been the emergence, for unknown reasons, of multiple strains and phage types of SE capable of infecting the reproductive tissues of egg-laying hens, without causing morbidity to the host, and without reducing egg production rates. Second is the fact that commercial egg handling practices have changed very little over the last 40 years. Inconsistent and poorly controlled egg handling, shelf-life dating, and transport practices permit eggs stored for extended periods of time at varying temperatures to be marketed to the consumer. In Europe, eggs are typically distributed and marketed at ambient temperatures. Eggs handled in this manner may contain elevated levels of SE bacteria relative to fresh eggs stored under refrigeration throughout their shelf-life. Third is the now well-established fact that many traditional egg cooking procedures are inadequate to assure the inactivation of SE in eggs and egg-containing foods (Saeed and Koons, (1993) J. Food Protect 56:927). As a result of these concerns, the 1997 edition of the Food Code strongly recommends the use of pasteurized eggs and/or egg products in higher-risk recipes, and whenever highly susceptible populations (preschool children, the elderly, the immunocompromised) are served recipes prepared using eggs.
Pasteurization of shell eggs has been a challenge for the egg processing industry because of the physical structure of the intact egg and the susceptibility of egg proteins to heat denaturation. Egg proteins are sensitive to heat treatment, which may result in a loss of egg functionality (e.g., impairment in egg quality and properties), protein coagulation, or even partial cooking of the egg. The albumen proteins are particularly susceptible to heat damage (R. P. Elliott et al., (1980) Eggs and egg products. In Microbial Ecology of Foods, Vol. II, Food Commodities. International Commission on Microbiological Specifications for Foods. pp. 521-566. New York: Academic Press).
Typical pasteurization methods employ hot aqueous solutions, hot oil, or hot dry air to apply heat to the outside of the shell egg, such that the heat is then transmitted to the inner contents of the egg. Thus, there is a lag in the heating of the inner layers of the egg as compared with the outer layers. Accordingly, to impart a sufficient thermal treatment (i.e., time and temperature) to the center of the yolk to achieve Salmonella kill the albumen must be subjected to a greater total thermal treatment than the yolk. It therefore requires careful optimization of processing conditions to achieve a sufficient level of Salmonella kill throughout the egg without impairing egg quality and functionality, in particular, the quality and functionality of the albumen.
In recent years, several research groups have attempted to develop methods to pasteurize the internal contents of shell eggs without significantly changing the appearance, color, texture, aroma or flavor of the eggs. It has been demonstrated that immersion heat treatments in hot aqueous solutions can be used to produce high-quality Salmonella-free shell eggs (J. D. Schuman et al., (1997) J. Applied Microbiology 83:438; PCT Publication No. WO 95/18538 to Vandepopuliere). These investigations identified time-temperature relationships necessary to inactivate Salmonella spp. within intact shell eggs, while preservingxe2x80x94to the greatest extent possiblexe2x80x94the appearance, flavor and cooking performance of unheated eggs. However, pasteurization methods employing liquid immersion or spray-washing of shell eggs are not permitted under certain regulatory schemes (for example, many European countries), thus creating a need for alternative methods of pasteurizing shell eggs.
Another research group has published the results of experiments designed to inactivate Salmonella enteritidis in intact shell eggs by (1) immersion in a pre-heated 57xc2x0 C. (134.6xc2x0 F.) water bath for up to 25 minutes, (2) placement of eggs in a pre-heated 55xc2x0 C. (131xc2x0 F.) dry hot-air oven with a forced air circulating fan for up to 180 minutes, or (3) use of a combination of methods (1) and (2), i.e., water-bath heating at 57xc2x0 C. (134.6xc2x0 F.) for 25 minutes followed by hot-air heating at 55xc2x0 C. (131xc2x0 F.) for 60 minutes (H. Hou et al., (1996) Food Microbiology 13:93). This group concluded that the combination method of heating in hot water followed by dry heated air (method (3) above) was the most commercially-feasible approach to pasteurizing shell eggs. PCT Patent Publication WO 97/02751 to Singh et al. (assigned to Purdue University) relates to this work and describes a method of heating intact eggs using a liquid or gas, or a combination of the two. A bulletin published by the Electric Power Research Institute (xe2x80x9cShell Egg Pasteurizationxe2x80x9d, Food Technology Alliance, 1997) describes the work by Hou and coworkers at Purdue using hot air to heat shell eggs.
The work of Hou et al., (1996) Food Microbiology 13:93 (described above) demonstrates problems associated with heating shell eggs in a hot dry air oven. The poor heat transfer to the eggs in a hot dry air process results in a protracted time to reach pasteurization temperatures (i.e., over an hour) and to achieve Salmonella kill (i.e., 3 hours). Such extended heating times are undesirable from a commercial processing standpoint. In addition, this method may also be objectionable with respect to sanitation, as subjecting eggs to elevated temperatures below the kill temperatures for pathogenic microorganisms may actually promote microbial growth. Moreover, a prolonged hot-air heating process can produce drying and coagulation of the outer and inner-shell membranes and the outer-most layers of the albumen, resulting in the formation of an objectionable hardened and dried layer adjacent to the shell.
It has been reported that Pasteurized Eggs (Laconia, N.H.) has developed a process for heating eggs that involves a series of water baths heated to between 62xc2x0 C. (143.6xc2x0 F.) and 72xc2x0 C. (161.6xc2x0 F.) (A. Coghlan, (1998) New Scientist, 2134:17; see also xe2x80x9cL""Oeuf Pasteurise Arrivexe2x80x9d, Le Figaro, May 25, 1998, page 19). The specific temperatures of the water baths and the residing times for the eggs in each bath are not disclosed. A final cold water bath is used to chill the eggs to refrigeration temperature. This report further notes that the Pasteurized Eggs method cannot be commercially practiced in Europe, where it is forbidden to wash eggs intended for human consumption (i.e, immerse in a water bath or spray-wash with water).
There is a need in the art for effective and efficient methods of reducing or eliminating Salmonella in shell eggs by means other than immersion in aqueous solutions. As described above, however, methods employing hot-dry air are undesirable because of the prolonged heating times required and the potentially deleterious effects on egg quality and appearance (Reviews of the art are provided, for example, by H. Hou et al., (1996) Food Microbiology 13:93, and W. J. Stadelman et al., (1996) Poultry Science 75:1067).
U.S. Pat. No. 5,843,505 to Davidson describes a method of heating shell eggs to reduce Salmonella contamination therein. This method employs a xe2x80x9cfluid heat transfer mediumxe2x80x9d, which can be a gas, but is preferably an aqueous medium. Although this publication makes reference to the use of xe2x80x9cwater vaporxe2x80x9d to heat shell eggs (Col. 6, lines 25-26), it fails to provide the specifications for providing a commercially-feasible process for achieving a balance between Salmonella kill and preservation of egg functionality and appearance.
U.S. Pat. No. 5,700,504 to Hale describes a method of heating shell eggs prior to pasteurization by irradiation to prevent the deterioration in egg quality normally associated with irradiation (due to thinning of the albumen following irradiation). Hale provides that intact eggs can be heated by any suitable method, including xe2x80x9chot water baths, steam, radio waves, microwaves, and ohmic heating methods.xe2x80x9d (Col. 2, lines 50-55). The method of Hale, however, is primarily intended as a pre-treatment thermostabilization process and not as a pasteurization process.
Since 1996, M. G. Waldbaum Company (Minneapolis, Minn.), a subsidiary of Michael Foods Inc., has produced and marketed in-shell pasteurized eggs for the retail and foodservice markets. These eggs are pasteurized using a thermal treatment process involving liquid immersion. As described above, liquid immersion of shell eggs is prohibited under some regulatory schemes.
Accordingly, there remains a need in the art for alternative and improved methods of reducing or eliminating Salmonella spp. in shell eggs.
It is an object of the present invention to provide for the heat treatment of shell eggs without utilizing liquid immersion or spray washing.
It is a further object of the invention to provide an effective method of pasteurizing shell eggs.
It is still a further object of the invention to provide methods of pre-treating shell eggs to bring them to pasteurization temperature using humidity controlled air.
It is yet a further object of the invention to provide a method of cooling shell eggs without liquid immersion or spray washing.
It is another object of the invention to provide a method of sanitizing the surface of a shell egg in the absence of liquid immersion or spray washing.
It is still a further object of the invention to reduce Salmonella spp. (e.g., Salmonella enteritidis) within shell eggs without substantially or significantly decreasing the quality or functional properties of the treated egg.
It is yet another object of the invention to produce a pasteurized shell egg that retains high quality and functional properties.
These and other objects of the invention are provided by the methods and apparatus described below.
The present invention is based, in part, on the discovery that efficient heating of shell eggs may be achieved using convection currents of humidity controlled air. Surprisingly, heating profiles for shell eggs achieved with humidity controlled air according to the present invention are quite similar to those achieved by immersion heating of shell eggs in an aqueous solution (see J. D. Schuman et al., (1997) J. Applied Microbiology 83:438). Accordingly, the present invention provides a viable alternative for reducing or eliminating Salmonella spp. in egg processing operations, in particular, in those situations in which pasteurization of shell eggs by liquid immersion is prohibited or otherwise undesirable. The humidity controlled air heating process described herein provides an efficient and attractive means of pre-heating and/or pasteurizing (i.e., pre-heating and holding) intact eggs. As further aspects, the present invention also provides methods for cooling shell eggs as well as surface sanitizing shell eggs using humidity controlled air.
These and other aspects of the present invention are set forth in more detail in the description of the invention below.